Add Javadoc since for GrpcObservationDocumentation.GrpcServerEvents.CANCELLED (#5483)

Author: izeye

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/grpc/GrpcObservationDocumentation.java

@@ -122,6 +122,10 @@ public String getName() {
             }
 
         },
+        /**
+         * For a canceled event.
+         * @since 1.14.0
+         */
         CANCELLED {
             @Override
             public String getName() {